The present invention relates to a method for laser printing and to a device for the implementation thereof.
Ink printing is used in many fields to reproduce complex designs. The printing of elements can thus notably be implemented in fields as varied as biology, electronics, materials or clockmaking. The problems met in these fields are similar and relate in particular to needs for making elements combinations on a very small scale. A reproduction of patterns which consists in depositing material at specific locations can be chemically or physically performed by using masks or resorting to a step of selective ablation.
To overcome the disadvantages of these methods (risk of contamination, complex implementation, difficult combination of the deposition of several elements), ink printing methods have been developed. These have the advantage of being adapted to very freely generate element patterns using computer-aided design tools which they are associated with.
In the field of biology, depending on the works, such printing methods are called bio-printing, micro-printing of biological elements or simply bio-printing. According to these methods, the biological tissue is obtained by printing droplets of bio-inks. In order to reach some volume, the droplets are arranged in layers which are stacked on each other.
In a first embodiment, ink is stored in a tank and goes through nozzles or capillaries in order to form droplets that are transferred onto a substrate. This first alternative solution, so-called a nozzle printing includes bio-extrusion, ink-jet printing or micro-valves printing. Bio-extrusion makes it possible to obtain a significant cell density of the order of 100 million cells per milliliter and a resolution of one millimeter. Micro-valves printing makes it possible to obtain a lesser cell density of the order of several million cells per milliliter and a better resolution of the order of 100 μm. Ink-jet printing makes it possible to obtain a cell density identical with that of micro-valves printing, of less than 10 million cells per milliliter and a better resolution of the order of 10 μm.
In the case of bio-extrusion, the cells are deposited from a first nozzle and a hydrogel is simultaneously deposited from a second nozzle. As an alternative solution, the cells and the hydrogel are mixed in a tank prior to the extrusion. In the two other cases, ink is an aqueous medium containing the cells. According to the alternative solutions, bio-extrusion makes it possible to deposit the ink continuously as filaments or discontinuously as droplets.
According to such nozzle printing modes, as the printing resolution is linked in particular to the nozzle section, only bio-inks with given rheological characteristics can be used for high resolutions. Bio-inks with high cell density can thus be printed with difficulty, with a high resolution since such printing technique induces, upon the passage of ink through the nozzle, high shear stresses liable to damage the cells. Besides, with this type of ink, the risk of nozzles being blocked by the cells is important mainly because of the sedimentation of cells inside the tanks.
A method for printing biological elements by laser has been developed in order to be able to use a wide range of bio-inks and achieve high resolution. This printing method, called laser bio-printing, is also known as “Laser-Assisted Bio-printing” (LAB). The invention specifically relates to this type of printing methods. For comparison, bio-laser printing makes it possible to print inks with a high cell density of the order of 100 million cells per milliliter with a resolution of 10 μm. Similarly, laser printing has also been developed in other fields in order to improve resolution and expand the range of usable inks.
Compared to the nozzle printing techniques, laser printing provides greater flexibility in use (ability to print on soft, uneven surfaces . . . ), reduces shear stress, and limits the risk of settling. According to another advantage, it is possible to print from a small volume of ink of the order of a few microliters, which is interesting for deposits of expensive materials. Eventually, it is possible to use the printing system to view and select the drop region as described in WO2011/107599.
As illustrated in FIG. 1, a device for laser printing biological elements which is based on the so-called “Laser-Induced Forward Transfer” (LIFT) technique, comprises a pulsed laser source 10 emitting a laser beam 12, a system 14 for focusing and orienting the laser beam 12, a donor substrate 16 which comprises at least one bio-ink 18 and a receiving substrate 20 so positioned as to receive droplets 22 emitted from the donor substrate 16. According to this printing technique, the laser beam is pulsed and a droplet is generated on each pulse.
Bio-ink 18 comprises a matrix, for example an aqueous medium, wherein elements are present, for example, cells, to be deposited onto the receiving substrate 20. The donor substrate 16 comprises a blade 24 transparent to the wavelength of the laser beam 12 which is coated with an absorbent layer 26 whereon bio-ink 18 is affixed as a film. The absorbent layer 26 makes it possible to convert light energy into kinetic energy. The laser beam 12 thus produces a punctual heating at the absorbent layer 26 that generates, by vaporization, a gas bubble 28 which, by expansion, causes the ejection of a droplet 30 of bio-ink.
According to a known arrangement, the laser beam 12 impacts the donor substrate 16 by being oriented in an approximately vertical direction and in an upward direction, or in the same direction as the gravitational force G. Bio-ink 18 is thus placed under the blade 24 so as to be oriented downwards, towards the receiving substrate 20 which is placed under the donor substrate 16. Given this arrangement, bio-ink 18 is in the form of a film with a thickness E lower than a given threshold to be held on the blade. This threshold varies in particular according to the surface tension, viscosity and density of bio-ink.
The formation of droplets 30 from the ink film depends on many parameters which relate in particular to the laser beam 12 (wavelength, energy, pulse duration, . . . ), the nature of the bio-ink 18 (surface tension, viscosity, . . . ), to external conditions (temperature, humidity, . . . ). The formation of droplets 30 also depends on the thickness E of the bio-ink film. The droplets will not form if the thickness E of the bio-ink film is not included in a thickness range defined by a lower bound and an upper bound. If the thickness E has a value above the upper limit, no droplet will form because the expansion of the gas bubble 28 is too weak to reach the free surface of the film. If the thickness E has a value under the lower limit, the gas bubble will burst 28 at the free surface causing the uncontrolled projection of a plurality of micro-droplets toward the receiving substrate.
Therefore, the film thickness E has to be substantially constant over the entire surface of the donor substrate 16 to obtain reproducibility of the droplet formation whatever the region of the donor substrate 16 affected by the laser beam 12. Now, as illustrated in FIG. 1, this thickness E is not constant. This reproducibility issue is not limited to the case of bio-inks. It is present whatever the field of use, during the laser printing of an ink film.
To remedy this problem, a publication entitled “Microdroplet deposition through a film-free laser forward technique” published on Oct. 1, 2011 on the site www.elsevier.com provides for a device as described in FIG. 2. As before, this device comprises a laser source 32 emitting a laser beam 34, a system 36 for focusing and orienting the laser beam 34, a donor substrate 38 which contains at least one bio-ink 40 and a receiving substrate 42 positioned to receive droplets 44 emitted from the donor substrate 38. According to this publication, the donor substrate 38 includes a tank 46 with no upper wall so that the free surface 48 of the bio-ink 40 contained in the tank faces the receiving substrate 42. To obtain a regular, substantially planar, free surface 48 bio-ink is not a thin film but a volume having a depth of the order of 3 mm. Thus, the tank bottom has no influence on the shape of the free surface 48 of the bio-ink and the side walls of the tank have a limited effect at the periphery of the free surface 48 because of the surface tension.
Given the depth of the volume of bio-ink, the free surface 48 is necessarily directed upwards to stay in the tank and the receiving substrate 42 is positioned above the bio-ink 40. According to this document, to obtain the ejection of a droplet, the laser beam 34 is focused just below the free surface 48 and has a depth of the order of 40 to 80 μm. Thus, the droplets emitted from the free surface 48 are projected toward the receiving substrate 42 in a direction of movement opposite the direction of the gravitational force G.
Although the solution proposed by this publication makes it possible to obtain a flat free surface 48 for the ink, it is not necessarily adapted to inks in the form of suspensions, such as bio-inks. As a matter of fact, as indicated above, such bio-inks contain elements to be printed, such as, for instance cells, embedded in a matrix, which tend to settle down on the tank bottom. As the concentration in elements to be printed is low near the free surface, the printed droplets have de facto low concentrations in cells, which is generally detrimental to the printed biological tissue. Besides, according to this method, the number of cells and the concentration in deposited cells can hardly be controlled.
Such settlement issue is not limited to bio-inks. Thus, it is found during the laser printing of inks as suspensions, such as suspensions of particles or nanoparticles in a liquid matrix, whatever the field of application of such inks. According to another disadvantage of the method of the publication, the ink must be able to absorb the laser beam, which could limit the range of inks that can be printed using this technique.
The present invention therefore aims to remedy the drawbacks of the prior art by providing a printing method which makes it possible to print a wide range of elements with great accuracy. In particular, this method makes it possible to print a wide range of biological elements, specifically so as to obtain complex biological tissues. For this purpose, the invention relates to a method for printing with at least one ink, with said method comprising a step of focusing a laser beam so as to generate a cavity in an ink film, a step of forming at least one ink droplet from a free surface of the ink film and a step of depositing said droplet onto a depositing surface of a receiving substrate positioned at a given distance from the film, characterized in that the laser beam is oriented in the direction opposite the gravitational force, with the free surface of the film being oriented upwards towards the depositing surface placed over the ink film.
This configuration makes it possible, in particular, to obtain a substantially constant thickness E for the ink film, while limiting the occurrence of settling phenomena. Besides, it makes it possible to use a wide range of inks. The ink printed using the method according to the invention can be any liquid ink and can be a solution or a suspension.
Bio-inks, the inks used in electronics or watchmaking can be cited, among the usable inks. According to one application, the ink is a bio-ink.
According to another characteristic, the film has a thickness of less than 500 μm and/or has a dimension of the free surface of the film on film thickness ratio greater than or equal to 10. The distance separating the ink film and the depositing surface and/or the beam laser energy are preferably so adjusted that the kinetic energy of the droplet is almost equal to zero when the droplet contacts the depositing surface. Such characteristic limits the risks of damaging the elements (cells or other elements) contained in the droplet. According to one embodiment, the distance separating the ink film and the depositing surface is equal to 1 to 2 mm and the beam laser energy is so adjusted that the kinetic energy of the droplet is almost equal to zero when the droplet contacts the depositing surface.
According to another characteristic, the printing method comprises a preliminary phase of calibration of the laser beam energy. This calibration phase comprises a step of measuring an included angle of a deformation of the free surface of the ink film at a set time after the impact of the laser beam and a step of adjusting the laser beam energy as a function of the measured value of the included angle.
The laser beam energy is so adjusted that the included angle is less than or equal to 105°. In this case, the laser beam energy is sufficient to cause the formation of a droplet. The energy of the laser beam is advantageously so adjusted that the included angle is greater than or equal to a second threshold to obtain a kinetic energy almost equal to zero at the time when the formed droplet reaches the depositing surface.
For an ink, preferably a biological ink, film with a thickness of the order of 40 and 50 μm, the time for measuring the included angle is preferably of the order of 4 to 5 μs from the impact of the laser beam. Advantageously, the ink film has a thickness greater than 20 μm. For an ink which is in suspension with a high concentration in elements to be printed, the ink film preferably has a thickness ranging from 40 to 60 μm. Advantageously, in order to improve the accuracy of the deposition of the elements to be printed, the ink film 74 has a thickness E between 1.5D and 2D, with D being the diameter of the elements to be printed which have an approximately spherical shape or the diameter of a sphere in which at least one element to be printed is inscribed.
The invention also relates to a printing device for implementing the printing method of the invention. It comprises:                at least a pulsed laser source so configured as to emit a laser beam,        an optical system for focusing and orienting said laser beam,        at least one donor substrate which at least an ink film is attached to with a free surface, and        at least one receiving substrate comprising a depositing surface.The printing device is characterized in that the laser beam is oriented in the direction opposite the gravitational force and in that the free surface of the film is oriented upwards towards the depositing surface placed above the ink film.        